1. Field of the Invention
Generally, the present invention relates to the formation of integrated circuits, and, more particularly, to the formation of metallization layers including highly conductive metals, such as copper, embedded into a dielectric material having low permittivity to enhance device performance.
2. Description of the Related Art
In modern integrated circuits, minimum feature sizes, such as the channel length of field effect transistors, have reached the deep sub-micron range, thereby steadily increasing performance of these circuits in terms of speed and power consumption. As the size of the individual circuit elements is significantly reduced, thereby improving, for example, the switching speed of the transistor elements, the available floor space for interconnect lines electrically connecting the individual circuit elements is also decreased. Consequently, the dimensions of these interconnect lines have to be reduced to compensate for a reduced amount of available floor space and for an increased number of circuit elements provided per chip. In integrated circuits having minimum dimensions of approximately 0.35 μm and less, a limiting factor of device performance is the signal propagation delay caused by the switching speed of the transistor elements. As the channel length of these transistor elements has now reached 0.18 μm and less, it turns out, however, that the signal propagation delay is no longer limited by the field effect transistors, but is limited, owing to the increased circuit density, by the close proximity of the interconnect lines, since the line-to-line capacitance is increased in combination with a reduced conductivity of the lines due to their reduced cross-sectional area. The parasitic RC time constants, therefore, require the introduction of a new type of material for forming the metallization layer.
Traditionally, metallization layers are formed by a dielectric layer stack including, for example, silicon dioxide and/or silicon nitride, with aluminum as the typical metal. Since aluminum exhibits significant electromigration at higher current densities that may be necessary in integrated circuits having extremely scaled feature sizes, aluminum is being replaced by copper, which has a significantly lower electrical resistance and a higher resistivity against electromigration. For devices having feature sizes of 0.13 μm and less, it turns out that simply replacing aluminum with copper does not provide the required decrease of the parasitic RC time constants, and, therefore, the well-established and well-known dielectric materials silicon dioxide (k≈4.2) and silicon nitride (k≈7) are increasingly replaced by so-called low-k dielectric materials. However, the transition from the well-known and well-established aluminum/silicon dioxide metallization layer to a low-k dielectric/copper metallization layer is associated with a plurality of issues to be dealt with.
For example, copper may not be deposited in higher amounts in an efficient manner by well-established deposition methods, such as chemical and physical vapor deposition. Moreover, copper may not be efficiently patterned by well-established anisotropic etch processes and therefore the so-called damascene technique is employed in forming metallization layers including copper lines. Typically, in the damascene technique, the dielectric layer is deposited and then patterned with trenches and vias that are subsequently filled with copper by plating methods, such as electroplating or electroless plating. Also, the damascene technique is presently a well-established technique for forming copper metallization layers in standard dielectric materials, such as silicon dioxide. The employment of low-k dielectrics requires the development of new dielectric diffusion barrier layers to avoid copper contamination of adjacent material layers, as copper readily diffuses in a plurality of dielectrics. Although silicon nitride is known as an effective copper diffusion barrier, which may also efficiently avoid the diffusion of, for instance, oxygen into the copper-based metal, silicon nitride may not be acceptable in certain applications using low-k dielectric layer stacks, owing to its high permittivity and also reduced etch selectivity in sophisticated via etch processes. Therefore, presently, silicon carbide is considered as a viable candidate for a copper diffusion barrier. It turns out, however, that copper's resistance against electromigration strongly depends on the interface between the copper and the adjacent diffusion barrier layer, and, therefore, in sophisticated integrated circuits featuring high current densities, it is generally preferable to use up to 20% nitrogen in the silicon carbide layer, thereby remarkably improving the electromigration behavior of copper compared to pure silicon carbide.
With reference to FIGS. 1a-1e, a typical conventional process flow will now be described to explain the problems involved in forming a metallization layer including copper and a low-k dielectric in more detail. FIG. 1a schematically shows a cross-sectional view of a semiconductor structure 100, in which a low-k dielectric material is to be patterned in accordance with a so-called via first/trench last process sequence, which is presently considered as a very promising process scheme in patterning low-k dielectrics. The semiconductor structure 100 comprises a substrate 101 that may include circuit elements, such as transistors, resistors, capacitors and the like, and which may include a lower metallization layer 102 including a metal region 103 embedded in a dielectric material 104. Depending on the level of the lower metallization layer 102, the metal region 103 may comprise copper and the dielectric 104 may be a low-k dielectric, such as hydrogen-containing silicon oxycarbide (SiCOH). A barrier layer 105 formed of nitrogen-containing silicon carbide (SiCN), which also serves as an etch stop layer in the following etch procedure for patterning an overlying low-k dielectric layer 106, is formed on the metallization layer 102, thereby confining the metal region 103. The low-k dielectric layer 106 may comprise, depending on the process sequence used, an intermediate etch stop layer (not shown), which in many applications may, however, be omitted for the benefit of a reduced total permittivity. The low-k dielectric material in the layer 106 may be comprised of any appropriate low-k dielectric material, such as SiCOH and the like. A cap layer 108, for example comprised of silicon dioxide or provided as an anti-reflective coating (ARC), may optionally be located on the low-k dielectric layer 106 and may then serve as a stop layer in removing excess copper in a subsequent chemical mechanical polishing (CMP) process. A resist mask 109 including an opening 110 is formed above the optional cap layer 108.
A typical process flow for forming the semiconductor structure 100 as shown in FIG. 1a may comprise the following steps. After planarizing the lower metallization layer 102, the barrier/etch stop layer 105 is deposited, for example, by a plasma enhanced chemical vapor deposition (PECVD) from trimethyl silane (3MS) and ammonia (NH3) as precursor gases. Then, the hydrogen-containing silicon oxycarbide is deposited, wherein, if required, the optional etch stop layer may be formed when a first thickness corresponding to a trench depth is obtained during the deposition of the dielectric layer 106. Thereafter, the deposition of the layer 106 may be resumed to achieve the required final thickness of the layer 106. Next, the cap layer 108, if required, is deposited with a specified thickness. The cap layer 108 may help to substantially reduce any interaction of the low-k dielectric of the layer 106 with the overlying resist mask 109 and may serve as a CMP stop layer. Then, the resist mask 109 is patterned in accordance with well-established deep UV lithography techniques to form the opening 110, determining the dimensions of the vias to be formed within the dielectric layer 106.
FIG. 1b schematically shows the semiconductor structure 100 after an anisotropic etch process for forming a via 111 in the cap layer 108 and the dielectric layer 106. During the anisotropic etch procedure, the barrier/etch stop layer 105 exhibits a significantly lower etch rate than the surrounding dielectric layer 106, so that the etch process may be stopped in or on the layer 105, wherein the layer 105 comprised of SiCN may exhibit an enhanced etch selectivity compared to an SiN layer as may frequently be used for an etch stop layer. Thereafter, the remaining photoresist not consumed during the anisotropic etch process is removed by an etch step in an oxygen-containing plasma ambient. It should be noted that the etch stop layer 105 may not suppress oxygen diffusion into the metal region 103 during resist ashing and any other subsequent manufacturing processes as efficiently as an SiN layer and hence the copper integrity may be reduced.
FIG. 1c schematically shows the semiconductor structure 100 in an advanced manufacturing stage. The via 111 is filled with an organic anti-reflective coating material to form a via plug 114, while the organic material is also provided at the remaining surface of the structure 100 to form an anti-reflective coating layer 112 for the subsequent photolithography. Thus, the plug 114 and the anti-reflective coating 112 serve to planarize the topography of the semiconductor structure 100 prior to the formation of a further photoresist mask 113 including a trench opening 115. The via plug 114 and the anti-reflective coating layer 112 may be formed by spin-on techniques and the like, and the photoresist mask 113 may be formed by sophisticated lithography methods, as are well known in the art.
FIG. 1d schematically shows the semiconductor structure 100 after completion of the trench forming step and removal of the layer 112 and the plug 114. That is, a trench 117 is formed in the underlying cap layer 108 and the upper portion of the dielectric layer 106. The trench 117 may be formed on the basis of well-established etch techniques. Subsequently, the etch stop layer 105 may be completely opened. During this etch process, the degree of “over-etching” of the copper region 103 may depend on the uniformity of the previous etch process for forming the via opening 111 and thus on the selectivity of the etch stop layer 105, since a reduced etch selectivity may result in reduced thickness uniformity of the etch stop layer 105 within the via opening 111 after the via etch process. After etching through the etch stop layer 105 within the opening 111, a barrier metal and a copper-based metal may be filled in the trench 117 and the via opening 111.
FIG. 1e schematically shows the semiconductor structure 100 after completion of the above-described process sequence, thereby forming a metallization layer 130. The metallization layer 130 comprises the trench 117 and the via 111 filed with a copper-based metal 119, wherein a barrier metal layer 118 is formed on inner sidewalls of the trench 117 and via 111 and on the bottom surfaces thereof. A surface 120 of the metallization layer 130 is planarized to allow the formation of a further metallization layer.
Typically, the barrier metal layer 118 may be deposited by physical vapor deposition, such as sputter deposition, with a thickness that insures sufficient protection against copper out-diffusion and at the same time provides the required adhesion to the surrounding low-k dielectric material. Typically, tantalum or tantalum nitride may be used as material for the barrier metal layer 118. Subsequently, a copper seed layer is deposited to promote the subsequent deposition of the bulk copper by electroplating. Then, the excess copper is removed by chemical mechanical polishing, wherein the cap layer 108 may also be removed, at least partially, and may act as a stop layer, to reliably control the CMP process. Thereafter, a further etch top layer, such as the layer 105, may be deposited to confine the copper and provide a reliable etch stop during the formation of a subsequent metallization layer.
The electromigration of copper strongly depends on the characteristics of the interface to the surrounding material. It is therefore important to maintain the integrity of the copper-based metal at regions 121, in which the copper of the metal region 103 is in contact with the barrier/etch stop layer 105 or with an etch stop layer still to be formed on the metal 119. As previously explained, the SiNC material of the layer 105 may have, despite its superior behavior in view of etch selectivity and permittivity, a reduced diffusion blocking effect with respect to, for instance, moisture and oxygen compared to SiN. Hence, reduced performance of the metal region 103 or 119 may result, thereby compromising the overall performance of the device 100.
In view of the above problems, a need exists for a technique that maintains superior barrier characteristics while avoiding, or reducing the effects of, one or more of the problems identified above.